This application claims priority from Korean Patent Application No. 2003-6288, filed on Jan. 30, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.
Field of the Invention
The present invention relates to a solid electrolyte, a method for preparing the same, and a battery using the same. More particularly, the present invention relates to a solid electrolyte, which is improved in ionic conductivity and electrochemical stability, a method for preparing the same, and a lithium battery or a thin film battery using the same.
Description of the Related Art
Rapid advancement of computer technology and mobile communication has brought an information revolution, and thus, development of information equipment is rapidly advancing toward digitalization, miniaturization, and multimedia application. As energy sources for representative portable information equipment such as notebook computers, personal digital assistants (PDAs), and mobile communication terminals, lithium ion batteries with lightweight and high energy density have been mainly used.
As the miniaturization of electronic equipment continues, the sizes of secondary batteries tend to determine the sizes of systems for electronic equipment. As representative examples of such systems, there are complementary metal oxide semiconductors (CMOSs), smart integrated circuit (IC) cards, micro-sensors, and micro-robots. Judging from development tendencies of semiconductor process and micro electro mechanical system (MEMS) technology, it is anticipated that thin film batteries will be used as energy sources for new generation, small-sized systems.
FIG. 1 shows a schematic structure of conventional thin film batteries. Referring to FIG. 1, thin film batteries have a fundamental structure in which a cathode 12, an electrolyte 14, and an anode 13 are sequentially laminated in the form of thin films on a collector 11, and have a total thickness of about 10 μm. For these reasons, thin film batteries have the following advantages.
Since an anode is disposed near a cathode by deposition in the form of thin film, thin film batteries have a high current density and an excellent battery efficiency. Furthermore, due to thin film formation, a distance between ions decreases, and thus, ion transfer is facilitated. Therefore, the contents of reactants can be greatly reduced. Still furthermore, thin film batteries having suitable shapes and sizes for specific purposes can be easily manufactured. Therefore, thin film batteries are promising main power sources for driving ultra small-sized electronic devices, MEMS devices, and ultra small-sized sensors.
Since fabrication of thin film batteries is carried out in the same manner as a semiconductor fabrication process, thin film batteries can be mounted together with electronic circuits on semiconductor chips. Therefore, CMOS memory chips that use thin film batteries as backup power sources can be realized. Furthermore, since thin film batteries can be formed in disused spaces of electric equipment, a space utility efficiency can be optimized. Still furthermore, various types of battery packs with various voltages and capacities can be realized by a series or parallel interconnection of batteries using an appropriate design and etching process. Therefore, thin film batteries can be used for many various purposes.
Thin film batteries require Li+ ion conductors (electrolytes) of perfect solid phase, unlike conventional lithium ion batteries. There have been reported that LISICON, Li4SiO4—Li3PO4 solid solutions, Li2O—B2O3—SiO2, and Lipon (Lithium phosphorus oxynitride) are suitable electrolytes with regard to stability in an atmosphere.
Reports have been made on crystalline solid electrolytes with excellent Li+ ionic conductivity. However, inorganic compound-based solid electrolytes with complex crystalline structure generally exhibit amorphous properties when deposited in the form of thin films, and crystallization of such thin films requires heat treatment at a high temperature. Therefore, it is impractical to utilize inorganic compound-based solid electrolytes in thin film batteries.
Meanwhile, glass electrolytes that exhibit high isotropic conductivity at an amorphous state can be more easily fabricated in the form of thin films relative to crystalline electrolytes. Also, since ionic conductivity of glassy electrolytes continuously varies depending on their compositions, it is easy to adjust film compositions upon deposition. Meanwhile, since thin film batteries have an electrode-to-electrode distance as small as several micrometers, even electrolytes with a low ionic conductivity of 10−7 S/cm can be used to fabricate effective thin film batteries. Therefore, thin film batteries can solve problems of glassy solid electrolytes that exhibit relatively low ionic conductivity.
A currently most noteworthy solid electrolyte for thin film batteries is Lipon as disclosed in U.S. Pat. No. 5,338,625 to John B. Bates et al., titled “Thin Film Battery and Method for Making Same”. The Lipon is formed in the form of thin film by radio frequency sputtering of Li3PO4 target under nitrogen atmosphere. The Lipon thin film exhibits high ionic conductivity of 2(±1)×10−6 S/cm at room temperature. In particular, since the Lipon thin film forms a very stable interface with cathode or anode, battery deterioration during operation remarkably decreases. Therefore, it has been reported that the Lipon thin film satisfies most requirements for solid electrolytes for thin film batteries. However, the properties of the Lipon thin film greatly varies depending on process variables for thin film formation. For this reason, reproducibility becomes poor [P. Birke et al., Materials for thin film batteries for application in silicon technology, Solid State Ionics 93 (1997) 1-15], and thus, it is difficult to achieve mass production of the Lipon thin film.
Meanwhile, U.S. Pat. No. 4,184,015 discloses a solid electrolyte comprising a composition represented by the following formula:(B2O3, xM, yN)-aLi2O-bLiQ
wherein, M is selected from the group consisting of Al2O3, V2O5, P2O5, As2O5, and As2O3, N is selected from SiO2 and GeO2, Q is selected from the group consisting of F, Cl, Br, S, SO4, MoO4, WO4, N, and PO4, 0≦x≦0.35, 0≦y≦0.8, 0<a≦0.2, and b≦2a.
However, the above solid electrolyte exhibits good ionic conductivity only at a high temperature of 100° C. or above. For this reason, there is a problem in that a battery using the above solid electrolyte exhibits poor battery properties when operated at room temperature.
Therefore, in order to develop thin film batteries as energy sources for 21 century's leading ultra small-sized systems, development of new glassy solid electrolytes that exhibit high ionic conductivity at room temperature and can be substituted for Lipon is necessary.